The coagulation arm of hemostasis (the maintenance of vascular patency) is triggered in response to various stimuli and results in the explosive generation of thrombin. The cofactor proteins factor V and VIII act as receptors and activity modulators, forming the enzymatic complexes responsible for thrombin generation. The absence of either of these cofactors results in severe life threatening bleeding disorders, e.g. Hemophilia A. We have recently obtained crystals of human factor V, bovine factor Va, and recombinant human factor VIII. The factor Va crystals diffract to better than 3.8 E (home source) and preliminary data analysis indicates an orthorhombic unit cell (63.2 x 85.3 x 229.1 E). Extensive biochemical studies are available for each protein, however a complete understanding of their function requires knowledge of their structures.